This invention relates to printers such as thermal transfer type printers, and more particularly to a tray assembly of printers in which a sheet of printing paper is moved forward and backward over a roller platen several times to effect a full color printing.
Recently, thermal transfer type printers have come into wide use, in which a thermal head is pressed against a sheet of printing paper supported on a roller platen to transfer dye materials carried on an ink sheet or ribbon to the sheet of paper. Some of these thermal transfer type printers are capable of printing full color images; in such case, a sheet of printing paper must undergo several printing operations in each of which a different primary color is printed thereon so that several primary colors are superposed on each other to produce a full color image. In some of these full color printers, the sheet of paper undergoing printing operation is moved forward and backward several times over the roller platen to effect a superposition of several primary colors.
FIG. 1 shows a conventional printer disclosed in Japanese Laid-Open Patent Application No. 61-154866. The structure of this printer is as follows: To the main body 1 of the printer is detachably attached a paper cassette 3 accomodating sheets of printing paper 2 therein; the sheets of paper 2 are fed from the cassette 3 one by one. An ink sheet 4 carries dye materials thereon that are transferred to the sheet of paper 2a under the printing operation; to effect the transfer, a thermal head 5 is pressed against the roller platen 6 with the sheet of paper 2a and the ink sheet 4 interposed therebetween. The roller platen 6 opposes the thermal head 5 and rotates in the forward direction (i.e. clockwise as viewed in the figure) during a printing operation to move the sheet of paper 2a forward (toward left in the figure), and in the backward direction during the time when the sheet of paper 2a under printing is moved backward to the initial printing position. Sheets of paper which are fed manually are supplied from the manual feed tray 7 detachably attached to the main body 1 of the printer. Sheets of printing paper which have been printed with the primary colors are received in the discharge tray 8 which is also detachably attached to the main body 1 of the printer. A feed roller 9 rotates in the direction indicated by the arrow to feed the sheets of paper 2 in the cassette 3 one by one to the printing mechanism including the platen 6. The pair of forwarding rollers 10 are disposed between the feed roller 9 and the roller platen 6 to forward sheets of paper either from the cassette 3 or from the manual feed tray 7. A pair of discharge rollers 11 are disposed to discharge the sheets of paper which has finished printing of the primary colors to the discharge tray 8. The sheet of paper 2a under printing is moved forward during printing of a primary color into the front paper passage 12; it is moved backward into the return paper passage 13 when it is returned to the position at which the printing of the next color is started. The direction of the first guide plate 14 is determined according as a sheet of paper is being fed or moved backward by the roller platen 6 rotating in the backward direction. The second guide plate 15 is disposed in the front paper passage 12 to guide the sheet of printing paper 2a which has finished printing operations to the discharge tray 8.
Next, the operation of the printer of FIG. 1 is supplemented. When a sheet of paper is fed from the paper cassette 3 or the manual feed tray 7, the first guide plate 14 is changed-over to secure the passage for the sheet of paper to the roller platen 6 in response to a signal from a paper sensor (not shown); the sheet of paper is thus fed to the roller platen 6. Thereafter, the thermal head 5 is pressed on the roller platen 6, the sheet of paper 2a and the ink sheet 4 being interposed therebetween. Thus, images, such as characters and figures, are printed on the sheet of paper 2a wrapped around the roller platen 6 in response to a signal from a figure processing device implemented in a microcomputer (not shown); namely, the dye material carried on the ink sheet 4 is selectively transferred to the sheet of paper 2a in accordance with the signal from the figure processing device. During a printing operation, the sheet of paper 2a is moved to the left in the figure and the front end portion of the sheet of paper 2a is thus moved into the front paper passage 12.
In the full color printing, three primary colors, e.g. yellow, magenta, and cyan, must generally be superposed on each other on the sheet of paper 2a at predetermined densities to obtain desired colors; thus, the above printing operation must be repeated several times. As a result, when a printing operation of a first primary color is over, the thermal head 5 is raised from the roller platen 6, and, forwarding the ink sheet 4 to the inital position at which the front end of the next primary color lies thereon, the roller platen 6 is rotated in the reverse direction to move the sheet of paper 2a under printing backward into the return passage 13 to the position at which the printing of the next primary color is started. The printing of the next color is effected thereafter in a way similar to the printing of the first primary color described above. By the way, when the sheet of paper 2a under printing is moved backward, the first guide plate 14 is changed over the guide the end portion of the sheet of paper 2a to the return paper passage 13.
Thus, the printing operations are repeated a number of times equal to the number of primary colors; the sheet of paper 2a which has finished these printing operations is discharged by the discharge rollers 11 to the discharge tray 8, following the course determined by the changed-over second guide plate 15.
The tray portion of the above printer have the following disadvantages. Namely, the manual feed tray 7 and the discharge tray 8 form separate parts which are detachably attached to the main body 1 of the printer 1 independently of each other; thus, the attachment portions of these trays 7 and 8 have structural weakness, and are prone to failures. Further, the paper return passage 13 is formed within the main body 1 of the printer; thus, the length of the passage 13 is limited by the dimension of the main body 1; it is therefore difficult to form a return passage 13 long enough to accommodate sheets of paper of longer sizes.
In view of the above disadvantages of the trays of the printer of FIG. 1, a structure of the printer shown in FIG. 2 has been developed, wherein the manual feed tray 7 and the discharge tray 8 are formed integrally to enhance its structural strength; further, the paper return passage 13 is extended to the extension passage in a paper guide 18 formed between the trays 7 and 8. (In FIG. 2, the reference numerals 1 through 13 represent parts or portions similar to corresponding parts or portions in FIG. 1.) The trays 7 and 8 and the paper guide 18 constitute an integrally formed tray assembly.
The tray assembly of FIG. 2, however, also suffers from problems. Namely, the size of printing paper used in printing is not always the same; it may differ from one set of sheets of paper to next. The size of the manual feed tray used in each occasion must be adapted to these sheets of paper of various sizes. Thus, the whole tray assembly, including the discharge tray 8 and the guide 18, must be moulded separately according to the paper sizes. The necessity of moulding a number of whole tray assemblies adapted to different paper sizes results in a high cost of production.